The experiments proposed here will clarify the influence of daylength upon the CNS of a seasonal breeder, the golden hamster. We will test the hypotheses that photoperiod regulates sexual behavior through its control of opiate binding in specific regions of the limbic forebrain, and that individual cells within these areas have the potential to respond to combinations of melatonin, androgen, and/or endogenous opiates. We will use in vitro receptor autoradiography to determine whether photoperiodically-induced changes in opiate receptor concentrations in the medial amygdala are correlated with alterations in copulatory behavior, and whether these effects exhibit long latencies and photorefractory phases characteristic of reproductive responses to daylength. Infusions of physiological patterns of melatonin will determine whether the nightly duration of secretion of this hormone regulates opiate binding. in order to determine whether androgens can directly regulate neurons which bind opiates and vice versa, we will combine in situ hybridization for mu, delta, and kappa opiate receptor mRNA's with immunocytochemistry for androgen receptor (AR). The extent of intracellular colocalization will be determined in specific forebrain regions. We will ask whether the numbers of AR-ir cells or opiate receptor mRNA-positive cells, the quantity of opiate receptor mRNA per cell, and the degree of colocalization of these signals are regulated by photoperiod or androgens. Finally, we will combine immunocytochemical detection of AR, endogenous opiates, tyrosine hydroxylase and GnRH with in situ analysis of melatonin receptor mRNA in order to determine which cell types are likely to respond to the pineal hormone. These experiments will advance our understanding of seasonal changes in neuroendocrine function, elucidate the impact of the environment on opiate binding and opiate receptor gene expression, and clarify mechanisms of photoperiodic control of reproduction and behavior. Results will be relevant to syndromes of abuse and addiction, manipulation of fertility, and seasonal affective disorders.